There are many applications for a device which can accurately and quickly position an object at a preselected point within a defined three dimensional space. For many of these applications it is required that the machine not only be capable of placing the object at the preselected coordinates which correspond to the distance chosen but also that it be capable of repeating that position with the same degree of accuracy. It is further important that the same degree of accuracy be available near the upper and lower limits of the system's movement as well as near the central portion.
One application in which this type of accurate positioning is required is in making measurements of radiation for calibrating the dosages to be administered to a patient. In this particular application, a container of water is used to approximate the attenuation attributable to the human body and a probe is accurately positioned within the water at varying coordinates corresponding to the position of an organ or the like. A source of radiation is then directed at the probe for a specific period of time. The probe may then be used to measure the amount of radiation received and hence predetermine to a very accurate degree the amount of radiation that a body tissue at that same position will receive when exposed to the same radiation source. This calibration significantly enhances radiation therapy in that controlled and measured dosages may be administered to the specific tissue located at the specified position in the particular patient requiring the treatment.
In the prior art, there have been systems developed to achieve the positioning of a probe within a cubicle containing water. These prior art systems generally utilize one or more chain drive mechanisms for at least one direction of movement, with cable drives for the other directions of movements. Of these two types of drive mechanisms, the cable drive has been found to be much more accurate in both the initial movement of the system to a particular preselected position and also in repeating that preselected position after the system has moved away. Of course, chain drive mechanisms rely on a chain and sprocket assembly which is virtually impossible to align and operate in a reversing mode and over the full length of the system without some slack in the chain. As can be appreciated, any slack in the system results in inaccuracies in positioning. There is also a problem in any flexible drive chain as there is almost always a certain amount of play between the individual links of the chain as they are pivotally connected together. Thus, it has been found that a chain drive provides less than desirable accuracy for those applications where both initial and repeated positioning of an object at a preselected point is desired.
In the cable drives of the prior art, a drive motor drives a capstan with a shortened chain and sprocket with a drive cable wrapped at least partially around the capstan and threaded through a series of pulleys which connect the cable between the support and the movable parts of the system. Although many different cable stringing schemes are available in the prior art, most if not all of these suffer from some inaccuracy caused by a failure to maintain the cable in a straight line orientation between the reeling in and reeling out position on the pulleys, thereby resulting in parallel and perpendicular runs of the cable; and in equalizing the tension between the take-up and feed action of the capstan as the cable circulates through the system during movement. Thus, at one end of the system's movement a different tension may result in a different spacing between successive locations. Also, system operation results in varying stresses placed on varying portions of the cable which have a tendency to stretch the cable and destroy the system calibration and accuracy.
Applicant has succeeded in developing a system for accurately positioning an object at any preselected point within a defined three dimensional space which utilizes a cable drive for each direction of movement. Thus, applicant's positioning system is a significant improvement over the prior art in that no chain drive mechanism of any kind is required. Instead, a cable drive including a capstan and drive motor is provided for each of the direction of movement required to fully "map" each individual point located within the three dimensional space defined by the system. While different sorts of coordinate systems may be used to "map" this space, applicant's preferred embodiment uses the three perpendicular coordinates of the Cartesian coordinate system; or X, Y, and Z as they are commonly known.
Applicant has further enhanced his system by designing and developing a cable drive and pulley system where the capstan and cable have an equalized tension therebetween throughout the entire limits of cable movement and the cable is aligned in a straight line between the take-up and feed positions of the pulleys which results in parallel cable runs. By utilizing an equalized tension and parallel or perpendicular cable layout, the problems of cable stretching or binding experienced in the prior art cable drives has been eliminated. Indeed, in applicant's system it is not strictly necessary that pulleys be used but instead any posts or other pivot point could be used as long as the cable is maintained in parallel or perpendicular runs.
In applicant's preferred embodiment, the cable drives for the Y and Z axes are mounted in a single sub-assembly which is in turn supported for movement along the X axis. A Y carriage is supported perpendicularly from the Z axis and the probe is supported from the Y carriage. By cantilevering the Y support carriage from the Z axis, all of the cable drives and controls can be located at one side of the system. This provides obvious advantages for many applications as it reduces the amount of material in the system and the possibility of error and reflection of radiation. Virtually all of the components are made of Delrin.TM. or other type of translucent plastic with stainless steel support rods and Teflon.TM. linear bearings to withstand the continuous exposure to water and provide long and reliable service. The cable itself may be a rubber or plastic coated stainless steel cable which enhances the frictional contact with the pulleys and capstan drives. The drive motors may be digital stepping motors capable of being computer controlled and which provide a high degree of accuracy in positioning of the probe.
By using applicant's invention, extremely accurate positioning of a probe or other object can be achieved which has been heretofore unknown in the prior art. For example, applicant is presently constructing and supplying machines incorporating his invention which provide a positioning resolution to 0.1 millimeters in each direction of movement. It is expected that even greater accuracy could be achieved if required for a particular application other than that which applicant is presently supplying machines for. Applicant has broadly described his invention for purposes of illustration and it may be more fully understood by referring to the drawings and description of the preferred embodiment which follows.